The present invention relates in general to substrate manufacturing technologies and in particular to processing technologies that use non-magnetically confined plasma. More specifically, the invention relates to methods and apparatuses for sensing the failure of sustaining confined plasma, called un-confinement, in a plasma processing chamber.
In the processing of a substrate, e.g., a semiconductor substrate or a glass panel such as one used in flat panel display manufacturing, plasma is often employed. As part of the processing of a substrate in a plasma chamber for example, the substrate is divided into a plurality of dies, or rectangular areas, each of which will become an integrated circuit. The substrate is then processed in a series of steps in which materials are selectively removed (etching) and deposited (deposition) in order to form electrical components thereon.
In general, plasma chambers may confine plasma through the use of non-magnetic methods (e.g., quartz confinement rings, etc.) in order to minimize chamber wall contact. This is beneficial for reducing contamination levels and memory effects. For example, many surfaces within the plasma chamber are configured with plasma resistant materials (e.g., silicon, silicon carbide, silicon nitride, quartz, etc.) that help to minimize surface wear without substantially increasing contaminants that may affect the substrate. However, continued exposure to plasma tends to attack and remove the protective material, often resulting in surface particle contamination and hence lower substrate yields.
Referring now to FIG. 1A, a simplified diagram of a capacitively coupled plasma (CCP) processing system is shown. FIG. 1A and its counterpart FIG. 1B are simplified in that many components that are irrelevant to the invention herein have been omitted to avoid unduly cluttering up the more relevant features. A typical CCP configuration commonly consists of two electrodes separated by a small distance, operating in a manner similar in principle to a capacitor in an electric circuit. Powered electrode 102 is typically configured as a chuck, and, further, may be coupled to a set of RF generators [not shown] by transmission line 118, positioned in transmission line bore 119.
Powered electrode 102 may further include a bias compensation circuit [not shown], including a set of positive poles and a set of negative poles configured to provide an electrostatic clamping force in order to clamp substrate 104 to chuck 102. For example, a set of positive and negative voltages of +300V and −300V respectively supplied to these poles may provide a sufficient electrostatic clamping force, in order to clamp substrate 104 to chuck 102. A bias compensation circuit helps maintain a consistent clamping force across the substrate during plasma processing by providing a bias compensation voltage (Vb).
A second grounded electrode 106 is typically placed above plasma 122. The RF return path generally follows a path in the upper part of the plasma chamber, from grounded electrode 106, through chamber liner 114, ground extension 116, and along the inner surface of the transmission line bore 119. Certain etch applications may require the upper electrode to be grounded with respect to the lower frequency RF signal (e.g., 2 MHz). Another etch application may require the upper electrode to be grounded with respect to the higher frequency RF signal (e.g., 27 MHz and/or 60 MHz). Still another etch application may require the upper electrode to be grounded with respect to all of the RF signal frequencies (e.g., 2 MHz, 27 MHz, and 60 MHz). Grounded electrode 106 further may include a protective layer of perforated silicon that allow plasma gases to pass through into the plasma chamber from a gas distribution system. Furthermore, a substrate 104 is commonly positioned with edge ring 103 on chuck/RF-powered electrode 102.
In general, the magnitude of Vb, the bias compensation voltage, is proportional to the ratio of the ground surface area (normally the grounded electrode) to the surface area of the substrate:
                              V          B                =                              (                          Ground_Area              Substrate_Area                        )                    N                                    [                  Equation          ⁢                                          ⁢          1                ]            where N is typically between 1 and 4.
In addition, a confinement ring set 117 may be positioned between the CCP source and the plasma chamber wall, and may further be raised and lowered as required to isolate plasma over the substrate surface. Typically, the confinement ring set 117 is configured as a series of quartz rings positioned around a horizontal perimeter of a substrate, and further positioned in varying distances above the substrate along the vertical axis. In general, the thickness of each confinement ring, as well as the size of the gap between any two rings, is configured in order to optimize the particular plasma process and control the pressure within the plasma. In some configurations, the confinement rings are of differing diameter and thickness. For example, a confinement ring positioned closer to a substrate along the vertical axis, may be smaller in diameter to one farther away from the substrate. In general, the volume defined by the confinement ring set, the substrate and the powered electrode may be referred to as the plasma space within the plasma processing system.
Current trends in the semiconductor industry to further reduce substrate feature sizes, as well as the implementation of new-substrate materials, has challenged current fabrication technologies. For example, it is becoming increasing difficult to maintain the uniformity or process results on larger substrates (e.g., >300 mm). In order to achieve substantially vertical etch profiles and high aspect ratios, higher process power and lower pressure may be required. Consequently, the resulting plasma may be challenging to confine. Should the plasma become unconfined, substantial damage may occur to both, the processed substrate, as well as the plasma chamber itself, as a result of a substantial uncontrolled increase in voltage.
To minimize un-confined plasma damage a bias voltage threshold is established which is proportional to the steady-state bias voltage at the wafer in the confined case. If the measured bias voltage at the wafer exceeds that threshold, RF is shut off and plasma is terminated immediately. This shut-off method is making use of the fact that Vb significantly increases when the plasma is in the unconfined state due to the change in electrode area ratio. When unconfinement occurs, according to equation 1, the area of ground has increased from the area of electrode 106 in FIG. 1A to the area of the entire inside surface of the plasma reactor.
In existing art, however, it may be difficult to determine the correct threshold voltage since prior knowledge of the steady-state or safe Vb value is required. In many cases, especially when new recipes are developed, an up-front estimate of this threshold may be very difficult, often leading to sensing of either “false positives” if a bias voltage threshold was set too low, or non-detection and hence plasma chamber damage if the bias voltage threshold was set too high.
In view of the foregoing, there are desired methods and apparatuses for sensing un-confinement in a plasma processing chamber.